In digital communication, as a modulation and demodulation method for performing efficient data transmission, a quadrature amplitude modulation (QAM) method in which both of phase information and amplitude information are used for data identification is well-known. In recent years, as a demand for capacity enlargement in a communication system, there has been a demand for increasing the modulation multi-level number. However, there is a problem that increasing the modulation multi-level number may increase a transmission error probability caused by noise, and lower noise resistance. In particular, phase noise mainly caused by a reference oscillator (LO; Local Oscillator) in a transmission apparatus and in a receiving apparatus may increase uncertainty of phase information, and may considerably deteriorate a bit error rate (BER). In view of the above, in order to perform data communication with enhanced reliability by a multi-level QAM method in which the number of signal points is e.g. 256 or larger, it is necessary to estimate a phase error caused by the phase noise with high precision, and to compensate the phase error. Concurrently, it is necessary to improve resistance against an error caused by other factors such as thermal noise.
In the aforementioned technical field, there is known a demodulation apparatus, in which a phase error is compensated by a phase locked loop, and a QAM symbol demapping apparatus for outputting a bit sequence reflecting likelihood information, and an error correction decoder for inputting the likelihood information and performing an error correction process are provided in the post-stage of the phase locked loop to implement improvement of error resistance. PTL 1 describes an example of the QAM symbol demapping apparatus.
However, it may be impossible to obtain a sufficiently improved bit error rate depending on a magnitude of phase noise included in a baseband signal to be output from a detector, or due to deterioration of precision of phase detection resulting from thermal noise or the like. In view of the above, there is known a technique for improving precision of phase error compensation by adaptively adjusting a bandwidth of a loop filter in a phase locked loop. PTL 2, PTL 3, and PTL 4 disclose the aforementioned technique. Error resistance, however, may yet be insufficient.
Further, there is known a demodulation method, in which a smoothing phase locked loop (S-PLL), which is an improved phase locked loop, is used in order to improve precision of phase error compensation. For instance, NPL 1 and NPL 2 disclose a principle on phase noise compensation by averaging as described above.